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Visualizing the Universe, part IV: Time to Get a Move On

One of the first groups of stars to be recognized as an actual group -- not just a chance alignment -- is the Hyades in Taurus. This is the closest major open cluster to the Earth.

This is what the Hyades look like now. They are the roughly V-shaped cluster near the bright star Aldebaran. However, although Aldebaran rounds out the "V" nicely, it's not a member of the cluster.

The Hyades are close enough that we have reliable measurements of the distances of its member stars, and we also have good measurements of the velocities of the stars as well, so it's possible to calculate its motion over time.

However, it's much more fun to show it. Here's an animation of the Hyades spanning 2 million years -- from 1 million years ago, to 1 million years from now. The animation tracks the star Gamma Tauri, one of the brighter Hyades members.

A million years ago, the Hyades were much closer to the Earth, with many stars of the 2nd magnitude covering an area almost the size of Orion. It would have been a spectacular sight. Currently, the Hyades are moving away from the Sun, and in 1 million years, they'll be much less conspicuous than they are now.

During the animation, you can see another famous cluster -- the Pleiades, which are currently not far from the Hyades as seen from Earth, but quite a bit farther away in space. The Pleiades move slowly towards the south (bottom) of the animation over its 2 million year time frame. Here are the two clusters at the end of the animation, 1 million years from the present:

It's interesting to compare the two clusters, especially towards the end of the animation. At the end of a million years, the Hyades are actually almost as far away as the Pleiades. However, the stars in the Pleiades are much brighter intrinsically. The brightest stars in both clusters are all giants, much larger and more luminous than the Sun, but the Pleiades have especially young, luminous blue-white giants instead of the yellow and red giants in the Hyades. As a result, after 1 million years, the Pleiades are actually a little more conspicuous, despite still being somewhat farther away.